Time flies

Meaning:

You say "Time flies" when you are surprised to see minutes, hours, days, weeks, or years have passed very quickly.

Example:

A: Haven't we met each other for such a long time?

B: Yes, it's been 10 years since we met last time.

Time flies!

(After they talk together for 4 hours)

A: It's already midnight. We need to go home.

B: Is it already? Time flies!